1. Field of Invention
The present invention relates to a method of examining dynamic cardiac electromagnetic activity and a detection of cardiac functions using the results thereof. More particularly, the present invention relates to a method of examining the magnetocardiographic signals and a diagnosis of coronary artery diseases using the results thereof.
2. Description of Related Art
Each heart beat is originated from the development a small pulse of electric current that spreads rapidly in the heart and causes the myocardium to contract (depolarization and repolarization). The electrical currents that are generated spread not only within the heart, but also throughout the body, resulting in the establishment of electric potentials on the body surface, which are detectable as changes in the electrical potential with an electrocardiograph (ECG). A typical ECG tracing of a normal heartbeat (or cardiac cycle) consists of a P wave, a PR interval, a QRS complex, a ST segment, a Q-T interval, a T wave and a U wave. In brief, the P wave represents the wave of depolarization that spreads from the SA node throughout the atria; the QRS complex corresponds to the depolarization of the ventricles; the T wave represents the repolarization (or recovery) of the ventricles; the U wave, which normally follows the T wave, is not always seen and is thought to represent the repolarization of the papillary muscles or Purkinje fibers. The Q-T interval represents the time for both ventricular depolarization and repolarization to occur; the ST segment following the QRS complex is the time at which the entire ventricle is depolarized. Any normal or abnormal deflections recorded by the ECG depend upon the origin of this chain of electrical activity. Hence, via the measurements of electrical activity during a cardiac cycle, cardiac functions or pathologies can be investigated.
Although electrocardiograph (ECG) provides information related to cardiac electrical activity, the ECG signals crucially depend on the contact between the electrodes and the body. Further, in order to obtain two-dimensional signals via ECG, many electrodes need to be placed on the body, which can be impractical and may create interference between signals. Moreover, to obtain more insightful results, it is often required to perform exercise electrocardiography test, which may impose discomfort to the patient. Therefore, alternative methods that are electrode-free, contact-free and stress-free are being investigated.
Non-contact measurement technologies, such as thallium scan, computer tomography, nuclear magnetic resonance imaging, etc. have been developed, as a diagnostic tool for coronary artery diseases (CAD). However, these methods require the participants to the injection of isotopes or contrast medium, or the subjection to X-ray or magnetic field, which is invasive, uncomfortable and potentially dangerous for the participants.
Many studies have demonstrated the benefit of magnetocardiography (MCG) imaging over the existing methods for certain clinical evaluation of cardiac functions and pathologies. Magnetocardiography is a noninvasive, contact-free, risk-free approach by measuring the magnetic fields of the heart generated by the same electric current as the ECG and will be altered where the electrical currents in the heart are disturbed. Although both MCG and ECG measure the cardiac depolarization and repolarization patterns, MCG may detect depolarization and repolarization in a different manner.
The magnetic signals of a beating heart can transmit through the body of a study subject and be sensed by sensors configured in proximity to but not in direct physical contact with the body. Hence, the problems in skin-electrode contact arising in ECG can be obviated. Further, MCG is less affected by the conductivity variations caused by other organs or tissues such as lung, bone and muscles. Many studies have demonstrated that MCG is potentially beneficial in various clinical applications.
However, one difficulty in obtaining the magnetocardiac signals is the weakness of the signals, which are in the order of tens of pico-Tesla for human. The superconducting quantum interference devices (SQUIDs), which exhibit a noise level less than the magnetocardiac signals by 2 to 3 orders in magnitude, have been developed to record magnetocardiac signals with an improved spatial-temporal signal resolution and a higher signal-to-noise ratio. Currently, there are many commercially available SQUID systems for detecting magnetocardiac signals. Some of these systems, which are known as multi-channel SQUID systems, may consist of many independent SQUID sensors (for example, more than 50 SQUID sensors) to allow the measurement of two-dimensional magnetocardiac signals originating from various sites over the heart. From a magnetocardiography, parameters such as α angles, smoothness index, current dipole moments can be estimated. Some reports have suggested that these parameters can be used as indicators for diagnosing cardiac functions or pathologies. However, other studies have indicated that these parameters overlap between normal and abnormal hearts. Hence, the existing MCG parameters are not adequate, in terms of sensitivity and specificity, for diagnosing cardiac functions or pathologies.